This trade will tell you whether Trump won debate

US presidential candidate Donald Trump (R) and Mexican President Enrique Pena Nieto prepare to deliver a joint press conference in Mexico City on August 31, 2016.

Hedge funds may not like Donald Trump, but that doesn't mean they won't try to make some money off him. Investors are building a big short in the Mexican peso, which has become somewhat of a poster child
for the unconventional Republican candidate's anti-trade and anti-immigration stances.

Ahead of the first debate between Hillary Clinton and Trump, the peso was trading close to the record lows set last week. Investors in the futures market have been building what is likely now to be the biggest short position ever in the currency.

"The peso has been beaten down like a pinata ever since he's started showing some strength. The peso has been the primary expression of emerging markets nervousness about Trump coming to power," said Boris Schlossberg, managing director, foreign exchange strategy at BK Asset Management.

Last Wednesday's 19.9130 was the record low for the peso against the dollar. Here's what to expect if the pair cracks the psychological level of 20.